git-gui: Add a 4 digit commit abbreviation to the blame viewer

git-gui: Add a 4 digit commit abbreviation to the blame viewer

We now show the first 4 digits of each commit in the left most
column of our blame viewer, before the line numbers.  These are
drawn as the data becomes available from git-blame --incremental,
and helps the user to visually group lines together.

I'm using only the first 4 digits because within a given cluster
of lines its unlikely that two neighboring commits will have the
same 4 digit prefix.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>